1. Field of the Invention
The present invention relates to a radio receiver and a method of selecting antennas. More specifically, the present invention relates to a radio receiver specifically applicable to for example a radio local area network (LAN), and particularly to a diversity type of receiver for receiving radio packets. The present invention specifically relates to a method of selecting antennas applicable to a diversity type of radio receiver.
2. Description of the Background Art
A diversity receiver scheme for use in wireless or radio transmission systems is adapted to receive electro-magnetic waves with two or more antennas, while selecting appropriate one of the antennas so as to use electromagnetic waves caught by an antenna in a better receiving state. A radio packet receiver employing such a diversity system comprises a high frequency circuit section and a baseband processor section. The high frequency circuit section is provided with a selector switch for selecting radio receive signals caught by appropriate one of the antennas and an analog-to-digital (A/D) converter section for converting an output from the selector switch into a corresponding digital signal.
The selector switch periodically switches the antennas at a predetermined time interval to receive appropriate electro-magnetic waves. The A/D converter section includes an A/D converter for RSSI (Received Signal Strength Indicator) for use in measuring the strength of electromagnetic waves and another A/D converter for I/Q (In-phase/Quadrature) components. The A/D converter section feeds the thus converted digital signals to the baseband section. The baseband section comprises an RSSI power detector subsection, an I/Q detector subsection and an I/Q demodulator subsection.
The A/D converter for RSSI in the high frequency section feeds the converted digital signals to the RSSI power detector subsection. Also, the A/D converter for I/Q components in the high frequency section feeds the converted digital signals to the I/Q detector subsection and the I/Q demodulator subsection.
The I/Q detector subsection, once detecting data, enables the I/Q demodulator subsection to operate. The I/Q demodulator subsection calculates self-correlation on the data received from selected one of the antennas to then store the obtained correlation value. Subsequently, it changes over the selector switch so as to receive waves from the other antenna and then calculates a self-correlation value on the data received from the other antenna. The baseband section compares the calculated self-correlation value with the stored self-correlation value to select one of the antennas which supplies a larger value in order to use thereafter the selected antenna as a receiver antenna.
On the other hand, the signal circuitry for the A/D converter for RSSI and the RSSI power detector subsection is used to detect, when transmitting, whether or not other terminals emit electromagnetic waves. In other words, it is used to determine whether or not the apparatus is allowed to transmit.
More specific diversity systems are disclosed in U.S. Pat. No. 6,141,392 to Hoshikuki et al and U.S. Pat. No. 5,507,035 to Bantz et al. In Hoshikuki et al, the output of a reverse spread demodulation circuit is fed to a changeover control circuit, and the changeover control circuit selects a diversity receiver antenna in response to a correlation output generated when the preamble of a packet is received, while the change-over of the antennas is refrained from during receiving packets to maintain the state generating no bit error, thereby preventing deterioration in transmission efficiency in a multi-path environment.
Bantz et al discloses the control of a multi-path fading in a wireless communication system by dynamically combining a transmitter antenna diversity technique and a receiver antenna diversity technique with each other. Particularly in Bantz et al, there are disclosed antenna selection diversity and switching antenna diversity schemes. In the former scheme, a station is provided with a plurality of separate radio communication transceivers connected to respective, separate antennas spaced at a distance from each other corresponding to at least a fading coherence distance, and the antennas are selected after demodulation and packet buffering. In the latter scheme, a plurality of antennas are periodically switched at a given time interval, as with the system described above.
In the meantime, when receiving electro-magnetic waves in a radio communication environment, there may exist electromagnetic waves acting as interferential waves other than intended waves. In view of the effect of such interferential waves and the like, it is hardly determinable, in simple power detection by the RSSI power detector section, which one of the antennas connected is in its better receiver state. For the purpose of such determination, the diversity system is adapted, as described above, to calculate correlation values for each antenna in the I/Q demodulator subsection and compare the obtained correlation values with each other to thereby select appropriate antennas. However, the I/Q demodulator subsection has a number of operative circuits, which causes a significant amount of electricity to be consumed until an appropriate antennas is selected. In both Hoshikuki et al and Bantz et al, there is disclosed a method of selecting antennas in which one of a plurality of antennas is selected in the diversity to enable reception of better waves, while there is neither disclosure nor suggestion of reduction in electric consumption in selecting antennas.